1. Field of the Invention
The present invention relates to a display device capable of displaying accurately, particularly black.
2. Description of the Related Art
In recent years, a display device having a self light emitting element has been actively developed. A thin film transistor is often used in a pixel portion of the display device as a semiconductor element. Light emission of the self light emitting element is controlled by turning on/off the thin film transistor. When the thin film transistor is off, a small current that is called an off current may flow. The self light emitting element emits light even with a small off current, which is easily recognized by the human eye and becomes a problem.
As a conventional method of reducing the off current, an LDD (Lightly Doped Drain) structure is known. In this structure, an LDD region doped with an impurity element at a low concentration is provided between a channel forming region and a source region or a drain region doped with an impurity element at a high concentration.
Also suggested is a so-called double gate structure having conductive films that are formed so as to overlap each other with a channel forming region interposed therebetween (see Patent Document 1). The Patent Document 1 discloses a method of reducing the off current by applying a constant voltage to the conductive film of the bottom layer. [Patent Document 1] Japanese Patent Laid-Open No. 2003-23161
However, when a thin film transistor has the LDD structure or the double gate structure as disclosed in the aforementioned Patent Document and the like, the number of manufacturing steps increases, which may decrease the yield.